An MRI apparatus is an apparatus which measures a nuclear magnetic resonance signal generated by applying a high-frequency magnetic field pulse to an object in a state where the object is disposed in a homogenous magnetostatic field and reconstructs an image of the object by an arithmetic operation of the nuclear magnetic resonance signal. A magnetic field generation device is used to generate a high magnetic field as the magnetostatic field in which the object is disposed, thereby obtaining an image having a high SN.
In recent years, with the development of a superconducting magnet, a high-magnetic field MRI apparatus which can realize a high magnetic field equal to or greater than 3 T has been in widespread use. In the high-magnetic field MRI, while a high SN is obtained, there is a problem in that irregularity occurs in an image during abdominal imaging or the like. One of the factors of irregularity is inhomogeneity of the magnetic field distribution (B1 distribution) of a high-frequency magnetic field pulse (also referred to as a transmission RF pulse), which excites an atomic spin in a tissue of an object. In general, since the resonance frequency of a high-frequency magnetic field for excitation is in proportion to magnetostatic field strength, it is necessary for the high-magnetic field MRI to irradiate a magnetic field at a higher frequency than a prior high-frequency magnetic field. In this case, the wavelength of the high-frequency magnetic field inside a living body has a scale comparable to the size of the living body (in particular, abdomen). For this reason, the phase of the high-frequency magnetic field changes depending on the position inside the living body, and the change appears as image irregularity.
As a technique for solving inhomogeneity of the high-frequency magnetic field, RF shimming is known. In the RF shimming, a transmission RF coil having multiple channels is used, and the strength and phase of an RF pulse provided to each of the channels is controlled separately, thereby reducing inhomogeneity of the B1 distribution. In order to determine the strength and phase of an RF pulse provided to each channel, the B1 distribution of each channel is required for each object and each imaging region, and various measurement methods of the B1 distribution are suggested.
A general method, which measures the B1 distribution, is called a Double Angle method (DAM) and measures B1 by an arithmetic operation of an image using an RF pulse at an arbitrary flip angle and an image using an RF pulse at a double flip angle (NPL 1). Furthermore, a method which takes the ratio of an image acquired immediately before pre-pulse application and an image acquired without pre-pulse application to compute the B1 distribution (NPL 2), or a method (Actual Flip Angle method: AFI) which acquires image data using a set of pulse sequences having different TR with RF pulses at the same flip angle and calculates the B1 distribution using the signal ratio of image data and the TR ratio (NPL 3) is suggested.